The present invention relates to an optical frequency modulation (FM) characteristics measurement apparatus for measuring optical characteristics of communication laser diodes.
The measurement apparatus of this invention is used for measuring optical FM characteristics of optical communication laser diodes. As shown in FIGS. 3A and 3B, varying laser diode (LD) currents shown in abscissas induce variations in the generated optical frequency (frequency modulation) and intensity (intensity modulation) shown in ordinates. An object of this measurement apparatus is to acquire a characteristics curve in which how frequency deviation (delta f) varies as a function of modulation frequency (fm) as illustrated in FIG. 3C. Other objects are to measure how frequency deviation varies as a function of a bias current of LD or other parameters. Until now, there is no measurement apparatus that satisfies these objects.
To acquire these characteristics curves, an engineer must fabricate by himself such a measurement apparatus. As a consequence, there are many problems in the conventional measurement, for example, (1) measuring the laser diode characteristics takes a long time because of manual operations, (2) fabricating the measuring apparatus requires hard and cumbersome work, (3) the measuring apparatus is not stable especially in setting a frequency discrimination point, and (4) as a result, measurement efficiency and accuracy are very low.
As noted above, there is so far no optical FM characteristics measurement apparatus which can easily measure an optical communication laser diode within a short period of time with high accuracy, high stability and ease of operation.
The prior art measurement apparatus which an engineer usually assembled in the past includes a Mach-Zehnder interferometer as a frequency discriminator which is formed of an piezo electric transducer and a polarization-maintaining optical fiber. This apparatus converts variation of optical frequency to variation of an optical intensity so that the resulted amplitude of the optical intensity can be converted to an electric signal. FIG. 4A shows a situation where a frequency modulation (FM) is converted to an intensity modulation (IM) by such an apparatus. As shown in FIG. 4A, according to the discrimination characteristics of the measurement apparatus, when the optical frequency varies, the output optical intensity varies accordingly. The output optical signal may be converted to an electrical signal by a photodiode (PD). Therefore, in this kind of apparatus, a frequency discriminating point must be set in the middle of the signal swing from the laser diode (LD) under test.
To measure LD characteristics accurately, the frequency discriminating point must be set in the middle of the signal and the discriminating point must be highly stable. In a prior art interferometer, the discriminating point is not stable enough because the discriminating point varies by, for example, a change in the angle of incident light or variation of ambient temperature, and etc.
FIG. 5A shows a block diagram of the prior art measurement apparatus having a Mach-Zehnder interferometer 3 as an optical frequency discriminator which is formed of an piezo electric transducer and a polarization-maintaining optical fiber. The apparatus also includes a voltage adjustment device 13 for adjusting the voltage level for the interferometer 3. The interferometer 3 in FIG. 5 is provided with an optical signal from a laser diode under test (not shown). The output signal from the interferometer is applied to a photo diode 4 wherein the optical intensity is converted to an electric signal to be analyzed by a test instrument such as a network analyzer 1. FIG. 5B shows a chart explanatory of the operation.
In FIG. 5A, the frequency discriminating point of the Mach-Zehnder interferometer 3 is set through the voltage adjustment device 13. In determining the frequency discriminating point, in the prior art, the maximum value O.sub.max of LD output is detected first by the photo diode 4, and then the discriminating point is set at the half value of the maximum value O.sub.max of the LD output as shown in FIG. 5B. Because operations are performed manually, to determine the frequency discriminating point will take a long time. Moreover, there are problems that output signal may have a large error because of the change of angle in the incident light to the Mach-Zehnder interferometer 3 and the discriminating point may not be maintained stable for a long time because of ambient temperature change.
Therefore, there is a need to provide an improved measurement apparatus which can easily measure an optical communication laser diode within a short time with high accuracy, high stability and ease of operation.